A radio frequency (RF) digital transmitter (DTX) for wireless communication has several advantages compared to the conventional wireless transmitters consisting of a digital baseband followed by an analog/RF front-end. The DTX shifts the digital-analog interface close to the antenna, and thus fewer analog components are involved. The DTX also enhances system flexibility through multi-mode and multi-band operation enabled by digital signal processing. Thus, the DTX has benefits for both wireless base-station and mobile applications.
In order to increase a wireless data rate and to improve network coverage with the efficient use of spectrum, concurrent multi-band (CMB) transmission methods have been developed. For example, the long-term evolution (LTE) communication standard defines non-contiguous inter-band carrier aggregation (IB-CA) for concurrently transmitting and receiving multiple disjoint frequency bands between a transmitter and a receiver. Recently, there have been growing interests on IB-CA DTX for non-contiguous CMB transmission, for which system designers are faced with significant challenges in experimental realization.
Delta-sigma modulation based CMB (CMB-DSM) technique allows IB-CA DTX to achieve an excellent dynamic range without spurious tones, but CMB-DSM technique suffers from a large amount of out-of-band noise.
Although a noise cancellation technique was discussed on an IB-CA DTX with non-contiguous CMB-DSM in U.S. Pat. No. 9,294,079, the suppression of out-of-band noise is not sufficient to eliminate the need of a high-order multi-band RF output filter (MB-RFOF) or significantly relax the MB-RFOF design requirements.
Accordingly, there is a need to realize a radio frequency (RF) digital transmitter with a noise canceller for sufficiently reducing the out-of-band noise without using MB-RFOFs.